1. Field of the Invention
The present invention relates to a motor driving circuit. More specifically, some preferred embodiment relate to a driving circuit for a linear driving type motor preferably for use in a personal computer.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
FIG. 5 shows an example of a conventional motor driving circuit. In this example, the motor 1 is a single-phase brushless motor equipped with a coil 2. This driving circuit includes a hall element 3, a hall amplifier 4, and an output amplifier 5. The motor 1 rotates in accordance with the driving signal S1 applied to the coil 2. The hall element 3 is configured to detect the rotating phase of the motor 1 and output the detection signal S2. The hall amplifier 4 is configured to amplify the detection signal S2 and output an amplified detection signal S3. The output amplifier 5 is configured to amplify the amplified detection signal S3 in accordance with a power source voltage V1 and output the driving signal S1. Thus, the motor 1, the hall element 3, the hall amplifier 4, and the output amplifier 5 form a feedback loop to linearly drive the motor 1.
In this motor driving circuit the revolving speed of the motor 1 varies depending on the power supply voltage V1. That is, the revolting speed of the motor 1 increases when the power supply voltage V1 is increased and decreases when the power supply voltage V1 is decreased.
In the aforementioned conventional motor driving circuit, even if the power supply voltage V1 is changed, the amplitude of the driving signal S1 is kept constant. The relationship between the power supply voltage V1 and the amplitude of the driving signal S1 is shown in FIGS. 6A to 6C.
FIG. 6A shows the waveform of the driving signal S1 in the case where the amplitude of the driving signal S1 is appropriate with respect to the power supply voltage V1. In general, the driving signal S1 is designed such that the apex of the waveform of the driving signal S1 is slightly distorted (saturated) by the power supply voltage V1.
FIG. 6B shows the waveform of the driving signal S1 in the case where the power supply voltage V1 is set higher than that in FIG. 6A in order to increase the revolving speed of the motor 1. As mentioned above, even if the power supply voltage V1 is changed, the amplitude of the driving signal S1 is kept unchanged. In this case, since the driving signal S1 is lower than the power supply voltage V1, the motor 1 cannot reach the set revolving speed due to the insufficient driving force. Furthermore, in the output amplifier 5, heat corresponding to the voltage difference between the power supply voltage V1 and the driving signal S1 will be generated, which requires heat releasing countermeasures.
FIG. 6C shows the waveform of the driving signal S1 in the case where the power supply voltage V1 is set lower than that in FIG. 6A in order to decrease the revolving speed of the motor 1. In this case, the apex of the waveform of the driving signal S1 is significantly distorted (saturated) at the region exceeding the power supply voltage V1 into an approximately rectangular waveform. As a result, the driving signal S1 changes rapidly, which in turn causes generation of noise.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.